Critical behavior at the nematic to smectic A phase transitions in commensurate and re-entrant incommensurate smectic phases.

Incommensurability in molecular length and in molecular pairing can lead to a wide variety of different liquid- crystalline mesophases. Transitions between these mesophases can be quite complex and involve the introduction of reentrancy of the nematic mesophase. In order to study these disorder-order disorder phase transitions, we have carried out high resolution x-ray scattering and heat capacity studies of the nematic (N) -> smectic A (SmA) transitions in 4'-(4"-n-alkoxybenzyloxy) -4-cyanostilbene with the alkyl chain of length 7(T7) and 8 (T8). The material T7 exhibits only a single nematic to smectic A sub 1 transition where the SmA sub 1 period is commensurate with the molecular length lambda.